QUANTIFYING THE BIOMOLECULAR FATE OF DIETARY C AND N IN EARTHWORM TISSUES AND CASTS

The importance of earthworms in the processing of soil organic matter (OM) was recognised by such illustrious scholars as Aristotle, who described them as 'the intestines of the Earth', and Charles Darwin who introduced the concept of earthworms as 'Nature's plough'. Nowadays, the capacity of earthworms to modify the distribution of organic compounds through the excavation of soils has led them to be described as 'soil engineers'. Their feeding activity in soils is widely recognised as beneficial, and results in increased soil quality through the addition of plant nutrients to mineral soil horizons and improved water retention, aeration, stability and workability. These improvements enhance root growth and increase biodiversity of soil fauna. Earthworms use soil organic matter, such as dead plant material, as a food source. Earthworms pull organic matter down into burrows, or simply eat it and subsequently deposit it in worm casts (their faeces) at or below the soil surface. During ingestion they transform their diet into small fragments. Some of these are digested and contribute to the earthworm's body mass, the rest being excreted in worm casts and mucus excretions. Surprisingly, it is unknown which components of organic matter earthworms eat, how much is absorbed into their body mass, and how much and which parts of their diet are excreted back into the soil. To find out which parts of organic matter earthworms eat, absorb and excrete, we are going to feed a common grassland, litter-feeding species of earthworm with 13C- and 15N-labelled ryegrass in soil mesocosms. 13C and 15N are naturally occurring non-radioactive stable isotopes of carbon and nitrogen that exist in much lower abundances in the atmosphere than the most abundant isotopes, 12C and 14N. The ratio between 13C:12C and 15N:14N can be calculated using isotope ratio mass spectrometry. The ryegrass for the experiment will be grown in sealed growth chambers: CO2 gas containing a high proportion of 13C will be supplied for photosynthesis, and nitrate (NO3-) containing a high proportion of 15N will be supplied in solution to the roots, producing 13C- and 15N-labelled plant material. When the earthworms eat the grass, the nutrients they absorb into their bodies, and then excrete as respired gas, mucus and casts, will also be labelled with 13C and 15N. We will extract different substances from the cuticle, muscle tissue, mucus and casts of the earthworms, and also from the remaining ryegrass and soil, to work out how much of the labelled substances have been utilised by the earthworms. We will also sample CO2 from the mesocosms to see how much of the diet has been used for energy production. By measuring how much of the 13C and 15N label has been incorporated into all of these samples, we will be able to calculate how much protein, carbohydrate and fat from the grass has been processed by the earthworm. Importantly, we will also be able to determine precisely if and how earthworms cause greater release (priming) or storage (sequestration) of carbon and nitrogen from organic matter in soils. Overall, this research will add a new dimension to the understanding of how earthworms affect the breakdown of organic matter, how much organic matter they can process, and whether their activity increases or decreases the amount of carbon and nitrogen stored in the soil.